1. Field of the Invention
The present invention relates to a backlight device used for illuminating a light transmission type of display apparatus from its rear face side, and a light transmission type of liquid crystal display apparatus using the same.
2. Description of the Related Arts
A liquid crystal display apparatus which is used as a display for a computer or television monitor uses a liquid crystal layer for modulating intensity of polarized light that is generated by light transmission through a polarizer, for example, such that, as illustrated by FIG. 13 where reference numeral 1 denotes a typical conventional liquid crystal display apparatus, light emitted from a light source 3 of a backlight device 2 is to be incident onto a side end face 4A of a light guide 4 in a substantially plate form, and to be outputted from a light output end face 4B which is shown as an upward end in the Figure, and the light emanated therefrom is to be diffused to an extent with a diffuser sheet 5 and to be converged with a prism sheet 6, so that the diffused light can illuminate, with directivity, a liquid crystal display panel 7 from its rear side.
In the backlight device 2, the source light which enters from the side end face 4A propagates by repeating total reflections in the light guide 4, and a part of the light is reflected by a light diffusing element 4D which is provided on an end face 4C arranged opposite to the output end face 4B and is emanated from the output end face 4B toward the liquid crystal display panel 7 through the diffusing sheet 5, while another part of the light in the light guide is emanated from the opposite end face 4C downward in the Figure and is reflected by a light reflection sheet 8 placed in the bottom to be returned into the light guide 4. The light diffusing element 4D may be substituted by fine rugged spots or by a large number of minute V-grooves provided on the surface and/or the rear face of the light guide 4.
The prism sheet 6 is a sheet on which a plurality of triangular unit prisms 6A (having a cross section of a triangle with or without a grounded vertex), or unit lenses (not shown in the Figure), each of which has a semicircular or semielliptical cross section, are arranged so that their ridgelines are parallel to each other.
The liquid crystal display panel 7 is constituted of a liquid crystal cell 7A and polarizers 7B and 7C, in such a way that the liquid crystal cell 7A has a liquid crystal layer (TN type or STN type of liquid crystal, liquid crystal for IPS or for VA, or the like) sandwiched in a pair of glass or plastic substrates (not shown), and that the polarizers 7B and 7C hold the outside faces of the liquid crystal cell (top and bottom faces in FIG. 13) between them.
Another conventional liquid crystal display apparatus 1A shown in FIG. 14 differs from the liquid crystal display apparatus 1 in FIG. 13 in that the prism sheet 6 in the backlight device 2A is arranged oppositely to that in the backlight device 2 in FIG. 13, that is, in FIG. 14, the unit prisms 6A arranged with their vertex downward, in the manner opposite to that of the unit prisms 6A in FIG. 13, and the diffuser sheet 5 is arranged on the upper side of the prism sheet 6. The light diffuser element 4D may be substituted by fine rugged spots or by a large number of minute V-grooves provided on the surface and/or the rear face of the light guide 4, in the same way as previously described for the light diffusing element 4D shown in FIG. 13.
The structure other than described above is the same as that in the liquid crystal display apparatus 1 in FIG. 13. Therefore, the same reference numerals are attached to the same elements in FIG. 14, and the explanation thereof is omitted.
The backlight device 2B in a further different conventional liquid crystal display apparatus 1B in FIG. 15 differs from the backlight device 2A shown in FIG. 14 in that the diffuser sheet 5 in the backlight device 2A shown in FIG. 14 is substituted by a second prism sheet 9 whose triangular prisms are directed perpendicular to those of the prism sheet 6.
The structure other than described above is the same as that in the liquid crystal display apparatus 1A in FIG. 14. Therefore, the same reference numerals are attached to the same elements in FIG. 15, and the explanation thereof is omitted.
In all of the liquid crystal display apparatuses 1, 1A, and 1B, the liquid crystal cell 7A is sandwiched between the polarizers 7B and 7C, and the polarizers 7B and 7C absorb about 50% of the incident light beams so that the efficiency for using the light (transparency) lowers; therefore, for obtaining sufficient brightness on the surface of the liquid crystal display panel 7, it is necessary to make more source light incident onto the polarizer 7B and consequently, not only higher power is consumed by the light source 3 in the backlight device but also practical problems, such as unclear display on the liquid crystal display panel 7, are caused because the heat from the light source 3 adversely affects the liquid crystal layer in the liquid crystal cell 7A.
In order to solve the above-mentioned problems, the following prior art invention for improving the efficiency in using light was proposed in, for example, Laid-Open Japanese Patent Application (JP-A-) Hei-3-45906: A polarized beam splitter sheet comprising a cholesteric liquid crystal layer is used to separate non-polarized light from a backlight device into two circularly polarized light beams which rotate oppositely to each other, and then one of the circularly polarized light beams is made incident on a liquid crystal display panel, and the other circularly polarized light beam is returned to the light source side so as to be introduced again into the polarized beam splitter sheet by such means as a curved mirror in the backlight device, for use to improve the light use efficiency.
In the invention disclosed in Laid-Open JP-A- Hei-6-324333, a non- polarized light beam from a light source is introduced to a diffuser device (which corresponds to a light diffuser sheet) and a beam converging element (which corresponds to a prism sheet), and then to a cholesteric filter where a reflected polarized beam is depolarized using the aforementioned diffuser device and again made incident on the cholesteric filter so as to be reused.
In the invention disclosed in Laid-Open Japanese Patent Publication (JP-P-) Hei-9-506985, a non-polarized light beam from a light source is introduced to an optical structure (which corresponds to a prism sheet) to be made incident on a reflection polarizer, and a polarized light beam which is reflected therefrom is reflected back by using a diffusing reflection surface so as to be reentrant on the reflection polarizer and reused.
The invention that is disclosed in JP-A- Hei-3-45906 does not utilize a prism sheet, and therefore, cannot be applied to such cases that a scattering light beam converged by a prism sheet is to be split by polarization by means of a cholesteric liquid crystal layer and has a demerit of a lower efficiency in reuse of light because the polarized light beam reflecting on a cholesteric liquid crystal layer is reflected back on a curved surface mirror.
The inventions which are disclosed in JP-A-Hei-6-324333 and in JP-P-Hei-9-506985 have such a problem that, when the brightness is measured using a conventional type of light absorbing polarizer which has a polarized beam splitting sheet arranged on the light output side (end far from a light source) of a prism sheet and a liquid crystal cell arranged further thereon, actually obtained brightness (light use efficiency) is only 1.6 fold whereas the theoretically expected value is 2 fold.